The invention relates to a safety switching apparatus for safely switching an electrical load, in particular an automated installation, on and off.
The invention further relates to a method for safely switching an electrical load, in particular an automated installation, on and off.
A safety switching apparatus within the meaning of the present invention is any switching apparatus which at least meets category 3, preferably even category 4, of the European standard EN 954-1 or a comparable safety standard. This includes in particular switching devices, safety controllers and sensor and actuator modules which are used for controlling and performing safety-critical functions in the sector of industrial production environments.
In this case, in particular switching devices are known which monitor the operating position of an actuating pushbutton, of an emergency off switch, of a protective door or of any other desired signaling device and disconnect a machine or a machine section as a function of this.
Failure of such safety switching apparatuses can have life-endangering consequences for the machine operating personnel, for which reason safety switching apparatuses are generally only used if they are permitted by the competent regulatory authorities (for example in Germany the Accident Prevention and Insurance Association, “Berufsgenossenschaft”).
An application of such a safety switching apparatus is provided, for example, in a two-hand switching device. The function of such switching device is typically to permit the activation of a machine or a machine section only when the operator depresses two pushbuttons. In this case, the pushbuttons are arranged, for example by them being spaced apart from one another in suitable fashion, in such a way that the actuation of the left and the right hand of the operator is required. This is intended to prevent the operator from activating the machine while one of his hands is still in the danger area of the machine.
In order to reduce the possibility of erroneous operation or manipulation of the device, two-hand switching devices are often equipped with a time monitoring apparatus. This time monitoring apparatus sets a further condition for the activation of the machine, namely that only a certain maximum duration should elapse between the actuation of the first and the second pushbutton, more generally the first and the second signaling element. If this maximum duration is exceeded, the machine is not activated even when both pushbuttons have been depressed. This is intended to prevent the operator from depressing the first pushbutton with one hand and impermissibly locking/clamping it and then actuating the second pushbutton with the same hand.
DE 42 15 327 C2 proposes a circuit for a safety switching apparatus. The principle of the two-hand switching system disclosed therein is based on the fact that in each case one capacitor is charged in two channels. If the first pushbutton is now pressed, a first relay is connected. At the same time, the charging operation of the second capacitor is ended by virtue of the capacitor being decoupled from one pole of the supply voltage and being connected to the other pole of the supply voltage via an adjustable potentiometer. In this way, a discharging process of the second capacitor is started. If the second pushbutton is now pressed, the energy remaining in the second capacitor is conducted to a second relay. If only a short amount of time has elapsed, the charge in the capacitor is sufficient for energizing/closing the relay. If, on the other hand, too much time has elapsed, i.e. the capacitor has been discharged to too great an extent, the energy is insufficient for closing the second relay. In this case, the load cannot be switched on.
However, one disadvantage of the proposed circuit is the fact that, under certain circumstances, it is possible for the load to be switched on in undesirable fashion for a short period of time. This is the case, for example, when the capacitor is discharged via the relay although the circuit for supplying the relay, at least temporarily, is not capable of holding the relay in the active position. In this case, the load is switched on and off over a short period of time, which is undesirable both from a safety point of view and from the point of view of the installation.
Furthermore, it is disadvantageous that, as a result of the indirect activation of the relay by means of the capacitor, it is always necessary to match the capacitor, the relay and the potentiometer to one another. In the event of changes in voltage and/or temperature, the matching process needs to be repeated regularly.
Electronic safety switching devices are also available on the market, such as, for example, the products PNOZ e2.1p and PNOZ e2.2p by the present applicant. Since these devices are entirely electronic, the control loops can be defined very precisely and the necessity for matching or calibration can be avoided. Since, in the case of purely electronic solutions, the flow of energy and the switching information are processed separately, however, they require an increased level of complexity in terms of component parts and software.
Against this background, one object of the present invention is to disclose a cost-effective safety switching apparatus and a method for safely switching an electrical load on and off, where the risk of faulty short-term switching-on is avoided and the time monitoring takes place more directly and is subjected to fewer fluctuations even in the case of changing ambient conditions.